1. Field of the Invention
The present invention relates to an optical modulation element for displaying gradation. More particularly, the invention relates to a liquid crystal element for displaying gradation using a liquid crystal having at least two stable states such as a ferroelectric liquid crystal.
2. Related Background Art
In the conventional liquid crystal television panel using an active matrix driving system, thin film transistors (TFT) are arranged in matrix for each of the pixels, and gate-on pulses are applied to the TFT to provide an on state between the source and drain of the TFT. Then, image signals are applied from the source and accumulated in a capacitor. In response to the accumulated image signals, the liquid crystal (twisted nematic: TN-liquid crystal, for example) is driven and at the same time, gradation is displayed by modulating the voltage of the image signals.
However, an active matrix driving television panel using a TN liquid crystal of the kind or an STN (supertwisted nematic) liquid crystal is as slow as several hundreds milliseconds in its response speed. Therefore, there is a problem to be encountered in using it for television driving of the ED or HD (high vision) for which future developments are anticipated.
Also, the ferroelectric liquid crystal (FLC) is being developed aggressively as a liquid crystal having spontaneous polarization for the purpose of using as a display element, light bulb, and others because of the higher responsiveness, memory capability, along with other advantages of this liquid crystal. Targets for the best use of the above-mentioned advantages include the optical shutter array, high-precision display element driven by a simple matrix, light bulb for high-density recording combined with a photoconductor, and potentially an active matrix driving animated image display using the thin film transistor (TFT) and others. The characteristics thereof are disclosed in U.S. Pat. No. 4,840,462, Proceeding of the SID, VOL. 30/2, 1989 [Ferroelectric Liquid Crystal Video Display] and others, for example.
Nevertheless, there are various difficult problems to be solved before the implementation of a gradation display using the FLC. Particularly, for the cells using the FLC, a passivation film is used for the prevention of the short circuit, which generates an inverse field. This inverse field produces an adverse effect on the gradation display.